dma-buf: Make .invalidate_mapping() truly optional

dma-buf: Make .invalidate_mapping() truly optional

The .invalidate_mapping() callback is documented as optional, yet it
effectively became mandatory whenever importer_ops were provided. This
led to cases where RDMA non-ODP code had to supply an empty stub.

Relax the checks in the dma-buf core so the callback can be omitted,
allowing RDMA code to drop the unnecessary function.

Removing the stub allows the next patch to tell that RDMA does not support
.invalidate_mapping() by checking for a NULL op.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20260131-dmabuf-revoke-v7-5-463d956bd527@nvidia.com